Devices such as portable telephones, digital cameras, PDAs, or touch panels and solar cells tend to spread more and more.
In those applications, resins such as acrylics have hitherto been used as protective members for protecting the displays. However, since acrylic resins have a low Young's modulus, there have been cases where the acrylic resin substrate bends when the display is pushed with, e.g., a finger, and comes into contact with the display to cause a display failure. There also have been a problem that the acrylic resin substrate is apt to be marred and to thereby impair visibility. One measure in overcoming those problems is to use a glass substrate as a protective member. Glass substrates for use as a protective member for those devices are required to (1) have high mechanical strength, (2) have a high Young's modulus, (3) have a low density, (4) be inexpensive and capable of being supplied in large quantities, and (5) have excellent bubble quality. For satisfying the requirement (1), glass substrates strengthened by ion exchange or the like (so-called strengthened glass substrates) have hitherto been used (see patent document 1 and non-patent document 1).
Patent Document 1: JP-A-2006-83045
Non-Patent Document 1: Tetsuro Izumitani et al., Atarashii Garasu To Sono Bussei (Novel glass and properties thereof), 1st edition, Management System Laboratory Co., Ltd., Aug. 20, 1984, pp. 451-498